7. IMPLICATIONS OF QUASISTELLAR-OBJECT ABUNDANCES

We can conclude from the previous sections that QSOs are associated
with vigorous star formation, consistent with the early-epoch
evolution of massive galactic nuclei or dense protogalactic clumps
(Section 5). However, QSO abundances provide
new constraints. For example, the general result for ZZ
suggests that most of the
enrichment and local star formation occur before QSOs turn on or
become observable. The enrichment times can be so short in principle
(Figure 13;
Hamann & Ferland
1993b) that the star formation might
also be coeval with QSO formation. In any event, the enrichment times
cannot be much longer that ~ 1 Gyr for at least the highest redshift
objects [depending on the cosmology
(Figure 1)].

If QSO metallicities are representative of a well-mixed
interstellar medium, we can conclude further that the star formation
was extensive. That is, a significant fraction of the initial gas
must be converted into stars and stellar remnants to achieve
ZgasZ. The
exact fraction depends on the IMF. A solar neighborhood IMF
(Scalo 1990, as in
the Solar Neighborhood model of Section 6)
would lead to mass fractions
in gas of only
15% at Z ~
Z and
would not be able to produce Zgas above a few
Z at
all. Flatter IMFs (favoring massive stars) could reach
ZgasZ while
consuming less of the gas. For example, the gas fraction when
Zgas ~
Z in the
Giant Elliptical model of Section 6.2 is
nearly 70%.

Figure 7 in
Section 2.6.3
illustrates the main
star formation characteristics required by the QSO data. The solid curves
on the right-hand side of that figure show theoretical BEL ratios
from photoionization simulations that use nominal BELR parameters plus
abundances from the two chemical-evolution models in
Figure 13 (see
Hamann & Ferland
1993b for more details). The evolution is
assumed to begin with the Big Bang, and the conversion of time into
redshift assumes a cosmology with Ho = 65 km
s-1 Mpc-1,
M = 1,
and = 0. (Lower
values of Ho or
M would
push the theoretical curves slightly toward the right
in that figure, for example by ~ 20% to ~ 50% in z if
M = 0.3,
see also Figure 1.) The main results
are that the Solar Neighborhood evolution is too slow and, in any case,
does not reach high enough metallicities or nitrogen enhancements to
match most
of the high-redshift QSOs. Much shorter time scales and usually higher
metallicities, as in the Giant Elliptical simulation, are needed.

A trend in the NV BELs suggests further that the metallicities are
typically higher in more luminous QSOs
(Section 2.6.3). That result needs
confirmation, but it could result naturally from a mass-metallicity
relationship among QSO host galaxies that is similar (or identical) to the
well-known relation in low-redshift galaxies
(Section 6.1;
Hamann & Ferland
1993b).
By analogy with the
galactic relation, the most luminous and metal-rich QSOs might reside
in the most dense or massive host environments. This situation would be
consistent with studies showing that QSO luminosities, QSO masses, and
central black-hole masses in galactic nuclei all appear to correlate
with the mass of the surrounding galaxies
(McLeod et al. 1999,
McLeod & Rieke
1995,
Bahcall et al. 1997,
Magorrian et al.
1998,
Laor 1998; see also
Haehnelt & Rees
1993). Direct
application of the galactic mass-metallicity relation suggests that
metal-rich QSOs reside in galaxies (or protogalaxies) that are minimally
as massive (or as tightly bound) as our own Milky Way.

One of the most interesting predictions from galactic studies
(Section 6.3) is that Fe /
ratios in QSOs might constrain the epoch of
their first star formation and perhaps the cosmology. In particular,
large Fe/ ratios (solar
or higher) would suggest that the local
stellar populations are at least ~ 1 Gyr old. At the highest QSO
redshifts (z ~ 5), this age constraint would
push the epoch of first star formation beyond the limits of current direct
observation, to z > 6
(Figure 1).
The 1 Gyr
constraint would also be difficult to reconcile with
M 1 in Big
Bang cosmologies (because the age of the universe at z 5 in this
cosmology is less than 1 Gyr). Conversely, measurements of low Fe /
would suggest that the
local stellar populations are younger than ~ 1 Gyr (although we could not
rule out the possibility that only SN IIs contributed to the enrichment
for some other reason). Some BEL studies have already suggested that Fe /
is above solar in
z > 4 QSOs
(Section 2.6.4), implying (albeit
tentatively) that these systems are already
1 Gyr old.

Quasar abundances should be viewed in the context of other measures of the
metallicity and star formation at high redshifts.
Damped-Ly absorbers
in QSO spectra, which probe lines of sight through large intervening
galaxies [probably spiral disks
(Prochaska & Wolfe
1998)]
have mean (gas-phase) metallicities of ~ 0.05
Z at
z 2
(Lu et al. 1996,
Pettini et al. 1997,
Lu et al. 1998,
Prochaska & Wolfe
1999).
The Ly forest absorbers,
which presumably probe much more extended and tenuous intergalactic
structures
(Rauch 1998),
typically have metallicities < 0.01
Z at
high redshifts
(Rauch et al. 1997,
Songalia & Cowie
1996,
Tytler et al. 1995).
The much higher metal abundances near QSOs are consistent with the rapid and
more extensive evolution expected in dense environments
(Gnedin & Ostriker
1997).
Perhaps this evolution is similar to that occurring
in the many star-forming objects that are now measured directly at
redshifts comparable to and greater than the QSOs (see references in
Section 5.1).

The detections of strong dust and molecular gas emissions from QSOs
support the evidence from their high abundances that considerable
local star formation preceded the QSO epoch. The dust and
molecules, presumably manufactured by stars, appear even in QSOs at
z 4
(Isaac et al. 1994,
Omont et al. 1996,
Guilloteau et al. 1997).